This invention relates to a cage valve for erosion control at the outlet, and to a method of reducing erosion control at the outlet of a cage valve.
A choke valve is a throttling device. It is commonly used as part of an oil or gas field wellhead. It functions to reduce the pressure of the fluid flowing through the valve. Choke valves are placed on the production “tree” of an oil or gas wellhead assembly to control the flow of produced fluid from a reservoir into the production flow line. They are used on wellheads located on land and offshore, as well as on wellheads located beneath the surface of the ocean.
Choke valves common to oil and gas field use are generally described in U.S. Pat. No. 4,540,022, issued Sep. 10, 1985, to Cove and U.S. Pat. No. 5,431,188, issued Jul. 11, 1995 to Cove. Both patents are commonly owned by Master Flo Valve, Inc., the assignee of the present application.
In general, choke valves involve:
a valve body having an axial bore, a body inlet (typically oriented as a side outlet to the axial bore) and a body outlet (typically referred to as an end outlet, aligned with the axial bore);
a “flow trim” mounted in the bore between inlet and outlet, for throttling the fluid flow moving through the body; and
means such as a stem and bonnet assembly for actuating the flow trim to open and close the choke valve, and for closing the upper end of the axial bore remote from the outlet.
There are four main types of flow trim commonly used in commercial chokes. Each flow trim involves a port-defining member forming one or more flow ports, a movable member for throttling the flow ports, and seal means for implementing a total shut-off. These four types of flow trim can be characterized as follows:
(1) a needle-and-seat flow trim comprising a tapered annular seat fixed in the valve body and a movable tapered internal plug for throttling and sealing in conjunction with the seat surface;
(2) a cage-with-internal-plug flow trim, comprising a tubular, cylindrical cage, fixed in the valve body and having ports in its side wall, and a plug movable axially through the bore of the cage to open or close the ports. Shut-off is generally accomplished with a taper on the leading edge of the plug, which seats on a taper carried by the cage or body downstream of the ports;
(3) a multiple-port-disc flow trim, having a fixed ported disc mounted in the valve body and a rotatable ported disc, contiguous therewith, that can be turned to cause the two sets of ports to move into or out of register, for throttling and shut-off; and
(4) a cage-with-external-sleeve flow trim, comprising a tubular cylindrical cage having ports in its side wall and a hollow cylindrical sleeve (external flow collar) that slides axially over the cage to open and close the ports. The shut-off is accomplished with the leading edge of the sleeve contacting an annular seat carried by the valve body or cage.
In each of the above, the flow trim is positioned within the choke valve at the intersection of the choke valve's inlet and outlet. In most of the valves, the flow trim includes a stationary tubular cylinder referred to as a “cage”, positioned transverse to the inlet and having its bore axially aligned with the outlet. The cage has restrictive flow ports extending through its sidewall. Fluid enters the cage from the choke valve inlet, passes through the flow ports and changes direction to leave the cage bore through the valve outlet.
The present invention is directed to the cage-with-external-sleeve flow trim type valve. External sleeve flow trim designs for choke valves are generally described in the prior art, see for instance U.S. Pat. No. 4,540,022, issued Sep. 10, 1985, to Cove et al., and U.S. Pat. No. 6,105,614, issued Aug. 22, 2000 to Bohaychuk et al., and US 2006/0163517 published Jul. 27, 2006 to Bohaychuk et al. A choke valve including an external sleeve flow trim in sub-sea applications is shown in U.S. Pat. No. 6,782,949 to Cove et al. These patents describe the beneficial characteristics of the external sleeve (also termed flow collar) design in erosion control, valve outlet erosion protection, seating integrity, and fluid energy control features.
A typical prior art choke valve (cage valve) is shown in FIG. 1. The prior art flow trim typically includes an external tubular throttling sleeve (flow collar) that slides over the cage. The sleeve acts to reduce or increase the area of the flow ports. An actuator, such as a threaded stem assembly, is provided to bias the sleeve back and forth along the cage. The rate that fluid passes through the flow trim is dependent on the relative position of the sleeve on the cage and the amount of port area that is revealed by the sleeve.
Maintenance on the deep sub-sea wellhead assemblies cannot be performed manually. An unmanned, remotely operated vehicle, referred to as an “ROV”, is used to approach the wellhead and carry out maintenance functions. To aid in servicing sub-sea choke valves, such choke valves have their internal components, including the flow trim, assembled into a modular sub-assembly. The sub-assembly is referred to as an “insert assembly” and is inserted into the choke valve body and clamped into position.
The choke valve “sees” or experiences relatively high and relatively low fluid pressures. More particularly, the fluid flowing in through the valve body inlet from the well has a high pressure. When the fluid passes through the restrictive cage flow ports it undergoes a considerable pressure drop. Thus, the fluid passing through the cage bore and the valve body outlet is at a lower pressure than that in the body inlet.